This invention relates generally to the method of current feedback taught in the above-referenced related patents that are directed to wideband directcoupled transistor amplifiers. These prior art amplifiers are sensitive to stray package and printed circuit board impedances which are unavoidably present at their inverting inputs. In the noninverting gain configuration the presence of capacitance Cp, as illustrated in prior art FIGS. 1 and 2 and as an example of parasitic or stray impedance, will cause peaking in the frequency response. From FIG. 2, the forward gain transfer function becomes vo(s)/vni(s)=(1+Rf/Rg)(1+sCpRp)/(1+Rf/A(s)), where Rp=RgRf/(Rg+Rf). The forward gain transfer function for the prior art amplifier of FIG. 1 is a similar expression. Due to the wide range of possible gain expressions A(s), using the zero term to cancel a pole of gain expression A(s) is not always possible. To minimize the frequency response peaking resulting from the zero term (1+sCpRp), either CpRp could be made very small or the pole term (1+Rf/A(s)) could be made large in order to insure that the bandwidth of the amplifier is much less than the zero frequency. The first option of increasing the zero frequency is not always desirable or possible due to constraints on the value of resistance Rp or the type of circuit or layout which gives rise to capacitance Cp. The second option is of couse undesirable because the bandwidth of the amplifier must be reduced. A third compensation option which is commonly used is to place a capacitance Cf in parallel with resistance Rf to create a pole which cancels the zero term above. Unfortunately, secondary pole and zero contributions at higher frequencies are also created by capacitance Cf, which may cause other undesirable effects. In any case, this in not a good general solution because as resistance Rg is varied to set the gain, capacitance Cf must vary to keep the product RgCp approximately equal to the product RfCf. This is both difficult and expensive due to the very small and possibly complex nature of the stray impedance.
Embodiments of the present invention as illustrated in FIGS. 3-4 and 7-9 completely eliminate the troublesome zero term caused by stray impedance at the inverting input and, furthermore, simplify the bias control circuitry of some known prior art amplifiers. In addition, the embodiments of the present invention illustrated in FIGS. 10 and 11 serve to reduce the inverting input bias currents of the amplifiers of FIGS. 3-4 and 7-9.